NOx treated mixed metal oxide catalyst

ABSTRACT

A catalyst comprising a mixed metal oxide is useful for the vapor phase oxidation of an alkane or a mixture of an alkane and an alkene to an unsaturated carboxylic acid and for the vapor phase ammoxidation of an alkane or a mixture of an alkane and an alkene to an unsaturated nitrile.

[0001] The present invention relates to an improved catalyst for theoxidation of alkanes or a mixture of alkanes and alkenes to theircorresponding unsaturated carboxylic acids by vapor phase catalyticoxidation; to a method of making the catalyst; and to a process for thevapor phase catalytic oxidation of alkanes or a mixture of alkanes andalkenes to their corresponding unsaturated carboxylic acids.

[0002] The present invention also relates to a method of producingunsaturated nitrites by subjecting alkanes or a mixture of alkanes andalkenes to vapor phase catalytic oxidation in the presence of ammonia.

[0003] Nitriles, such as acrylonitrile and methacrylonitrile, have beenindustrially produced as important intermediates for the preparation offibers, synthetic resins, synthetic rubbers, and the like. The mostpopular method for producing such nitrites is to subject an olefin suchas propene or isobutene to a catalytic reaction with ammonia and oxygenin the presence of a catalyst in a gaseous phase at a high temperature.Known catalysts for conducting this reaction include a Mo—Bi—P—Ocatalyst, a V—Sb—O catalyst, a Sb—U—V—Ni—O catalyst, a Sb—Sn—O catalyst,a V—Sb—W—P—O catalyst and a catalyst obtained by mechanically mixing aV—Sb—W—O oxide and a Bi—Ce—Mo—W—O oxide. However, in view of the pricedifference between propane and propene or between isobutane andisobutene, attention has been drawn to the development of a method forproducing acrylonitrile or methacrylonitrile by an ammoxidation reactionwherein a lower alkane, such as propane or isobutane, is used as astarting material, and it is catalytically reacted with ammonia andoxygen in a gaseous phase in the presence of a catalyst.

[0004] In particular, U.S. Pat. No. 5,281,745 discloses a method forproducing an unsaturated nitrile comprising subjecting an alkane andammonia in the gaseous state to catalytic oxidation in the presence of acatalyst which satisfies the conditions:

[0005] (1) the mixed metal oxide catalyst is represented by theempirical formula

Mo_(a)V_(b)Te_(c)X_(x)O_(n)

[0006] wherein X is at least one element selected from the groupconsisting of niobium, tantalum, tungsten, titanium, aluminum,zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium,nickel, palladium, platinum, antimony, bismuth, boron and cerium and,when a=1, b=0.01 to 1.0, c=0.01 to 1.0, x=0.01 to 1.0 and n is a numbersuch that the total valency of the metal elements is satisfied; and

[0007] (2) the catalyst has X-ray diffraction peaks at the followingangles (±0.3°) of 2θ in its X-ray diffraction pattern: 22.1°, 28.2°,36.2°, 45.2° and 50.0°.

[0008] Similarly, Japanese Laid-Open Patent Application Publication No.6-228073 discloses a method of nitrile preparation comprising reactingan alkane in a gas phase contact reaction with ammonia in the presenceof a mixed metal oxide catalyst of the formula

W_(a)V_(b)Te_(c)X_(x)O_(n)

[0009] wherein X represents one or more elements selected from niobium,tantalum, titanium, aluminum, zirconium, chromium, manganese, iron,ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony,bismuth, indium and cerium and, when a=1, b=0.01 to 1.0, c=0.01 to 1.0,x=0.01 to 1.0 and n is determined by the oxide form of the elements.

[0010] U.S. Pat. No. 6,043,185 also discloses a catalyst useful in themanufacture of acrylonitrile or methacrylonitrile by the catalyticreaction in the vapor phase of a paraffin selected from propane andisobutane with molecular oxygen and ammonia by catalytic contact of thereactants in a reaction zone with a catalyst, wherein the catalyst hasthe empirical formula

Mo_(a)V_(b)Sb_(c)Ga_(d)X_(e)O_(x)

[0011] where X is one or more of As, Te, Se, Nb, Ta, W, Ti, Zr, Cr, Mn,Fe, Ru, Co, Rh, Ni, Pd, Pt, B, In, Ce, Re, Ir, Ge, Sn, Bi, Y, Pr, analkali metal and an alkaline earth metal; and when a=1, b=0.0 to 0.99,c=0.01 to 0.9, d=0.01 to 0.5, e=0.0 to 1.0 and x is determined by theoxidation state of the cations present.

[0012] Unsaturated carboxylic acids such as acrylic acid and methacrylicacid are industrially important as starting materials for varioussynthetic resins, coating materials and plasticizers. Commercially, thecurrent process for acrylic acid manufacture involves a two-stepcatalytic oxidation reaction starting with a propene feed. In the firststage, propene is converted to acrolein over a modified bismuthmolybdate catalyst. In the second stage, acrolein product from the firststage is converted to acrylic acid using a catalyst composed of mainlymolybdenum and vanadium oxides. In most cases, the catalyst formulationsare proprietary to the catalyst supplier, but, the technology is wellestablished. Moreover, there is an incentive to develop a single stepprocess to prepare the unsaturated acid from its corresponding alkene.Therefore, the prior art describes cases where complex metal oxidecatalysts are utilized for the preparation of unsaturated acid from acorresponding alkene in a single step.

[0013] European Published Patent Application No. 0 630 879 B1 disclosesa process for producing an unsaturated aldehyde and a carboxylic acidwhich comprises subjecting propene, isobutene or tertiary butanol to gasphase catalytic oxidation with molecular oxygen in the presence of (i) acatalyst composite oxide represented by the formula

Mo_(a)Bi_(b)Fe_(c)A_(d)B_(e)C_(f)D_(g)O_(x)

[0014] wherein A represents Ni and/or Co, B represents at least oneelement selected from Mn, Zn, Ca, Mg, Sn and Pb, C represents at leastone element selected from P, B, As, Te, W, Sb and Si, and D representsat least one element selected from K, Rb, Cs and Tl; and

[0015] wherein, when a=12,0<b≦10,0<c≦10,1≦d≦10,0≦e≦10,0≦f≦20 and 0≦g≦2,and x has a value dependent on the oxidation state of the otherelements; and (ii) a molybdenum oxide which in itself is substantiallyinert to said gas phase catalytic oxidation to provide the correspondingunsaturated aldehyde and unsaturated carboxylic acid.

[0016] Japanese Laid-Open Patent Application Publication No. 07-053448discloses the manufacture of acrylic acid by the gas-phase catalyticoxidation of propene in the presence of mixed metal oxides containingMo, V, Te, O and X wherein X is at least one of Nb, Ta, W, Ti, Al, Zr,Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, Li, Na, K, Rb, Cs andCe.

[0017] Published International Application No. WO 00/09260 discloses acatalyst for selective oxidation of propene to acrylic acid and acroleincontaining a catalyst composition comprising the elements Mo, V, La, Pd,Nb and X in the following ratio:

MO_(a)V_(b)La_(c)Pd_(d)Nb_(e)X_(f)

[0018] wherein X is Cu or Cr or a mixture thereof,

[0019] a is 1,

[0020] b is 0.01 to 0.9,

[0021] c is >0 to 0.2

[0022] d is 0.0000001 to 0.2,

[0023] e is 0 to 0.2, and

[0024] f is 0 to 0.2; and

[0025] wherein the numerical values of a, b, c, d, e and f represent therelative gram-atom ratios of the elements Mo, V, La, Pd, Nb and X,respectively, in the catalyst and the elements are present incombination with oxygen.

[0026] Commercial incentives also exist for producing acrylic acid usinga lower cost propane feed. Therefore, the prior art describes caseswherein a mixed metal oxide catalyst is used to convert propane toacrylic acid in one step.

[0027] U.S. Pat. No. 5,380,933 discloses a method for producing anunsaturated carboxylic acid comprising subjecting an alkane to a vaporphase catalytic oxidation reaction in the presence of a catalystcontaining a mixed metal oxide comprising, as essential components, Mo,V, Te, O and X, wherein X is at least one element selected from thegroup consisting of niobium, tantalum, tungsten, titanium, aluminum,zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium,nickel, palladium, platinum, antimony, bismuth, boron, indium andcerium; and wherein the proportions of the respective essentialcomponents, based on the total amount of the essential components,exclusive of oxygen, satisfy the following relationships:

[0028] 0.25<r(Mo)<0.98, 0.003<r(V)<0.5, 0.003<r(Te)<0.5 and 0.003<r(X)<0.5, wherein r(Mo), r(V), r(Te) and r(X) are the molar fractionsof Mo, V, Te and X, respectively, based on the total amount of theessential components exclusive of oxygen.

[0029] Published International Application No. WO 00/29106 discloses acatalyst for selective oxidation of propane to oxygenated productsincluding acrylic acid, acrolein and acetic acid, said catalyst systemcontaining a catalyst composition comprising

Mo_(a)V_(b)Ga_(c)Pd_(d)Nb_(e)X_(f)

[0030] wherein X is at least one element selected from La, Te, Ge, Zn,Si, In and W,

[0031] a is 1,

[0032] b is 0.01 to 0.9,

[0033] c is >0 to 0.2,

[0034] d is 0.0000001 to 0.2,

[0035] e is >0 to 0.2, and

[0036] f is 0.0 to 0.5; and

[0037] wherein the numerical values of a, b, c, d, e and f represent therelative gram-atom ratios of the elements Mo, V, Ga, Pd, Nb and X,respectively, in the catalyst and the elements are present incombination with oxygen.

[0038] Japanese Laid-Open Patent Application Publication No. 2000-037623discloses a method for producing an unsaturated carboxylic acidcomprising subjecting an alkane to a vapor phase catalytic oxidation inthe presence of a catalyst having the empirical formula

MOV_(a)Nb_(b)X_(c)Z_(d)O_(n)

[0039] wherein X is at least one element selected from the groupconsisting of Te and Sb, Z is at least one element selected from thegroup consisting of W, Cr, Ta, Ti, Zr, Hf, Mn, Re, Fe, Ru, Co, Rh, Ni,Pd, Pt, Ag, Zn, B, Al, Ga, In, Ge, Sn, Pb, P, Bi, Y, rare earth elementsand alkaline earth elements, 0.1<a<1.0, 0.01<b<1.0, 0.01<c<1.0, 0<d<1.0and n is determined by the oxidation states of the other elements.

[0040] Despite the above-noted attempts to provide new and improvedcatalysts for the oxidation of alkanes to unsaturated carboxylic acidsand for the ammoxidation of alkanes to unsaturated nitrites, oneimpediment to the provision of a commercially viable process for suchcatalytic oxidations is the identification of a catalyst providingadequate conversion and suitable selectivity, thereby providingsufficient yield of the unsaturated product.

[0041] By the present invention, there are provided catalysts whereinthe performance is enhanced by treating mixed metal oxide catalystprecursors with a source of NO_(x) to form a treated admixture, andcalcining the treated admixture while the NO_(x) is present in theadmixture.

[0042] Thus, in a first aspect, the present invention provides a processfor improving the performance characteristics of a catalyst, comprisingthe steps of:

[0043] a) providing precursors for a mixed metal oxide having theempirical formula

A_(a)D_(b)E_(c)X_(d)O_(e)

[0044] wherein A is at least one element selected from the groupconsisting of Mo and W, D is at least one element selected from thegroup consisting of V and Ce, E is at least one element selected fromthe group consisting of Te, Sb and Se, and X is at least one elementselected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe,Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr,Be, Mg, Ca, Sr, Ba, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu;and a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0, and e is dependenton the oxidation state of the other elements;

[0045] b) adding a source of NO_(x) to said precursors to form anadmixture; and

[0046] c) calcining said admixture while said NO_(x) is present in saidadmixture.

[0047] In a second aspect, the present invention provides a process forproducing an unsaturated carboxylic acid, which comprises subjecting analkane or a mixture of an alkane and an alkene to a vapor phasecatalytic oxidation reaction in the presence of a catalyst containing amixed metal oxide having the empirical formula A_(a)D_(b)E_(c)X_(d)O_(e)wherein A is at least one element selected from the group consisting ofMo and W, D is at least one element selected from the group consistingof V and Ce, E is at least one element selected from the groupconsisting of Te, Sb and Se, and X is at least one element selected fromthe group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni,Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr,Ba, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu; and a=1, b=0.01 to1.0, c=0.01 to 1.0, d=0.01 to 1.0, and e is dependent on the oxidationstate of the other elements. The catalyst composition, while in itsprecursor state, is treated with an NO_(x) source, such as nitric acid,and calcined.

[0048] In a third aspect, the present invention provides a process forproducing an unsaturated nitrile, which comprises subjecting an alkane,or a mixture of an alkane and an alkene, and ammonia to a vapor phasecatalytic oxidation reaction in the presence of a catalyst containing amixed metal oxide having the empirical formulaA_(a)D_(b)E_(c)X_(d)O_(e),

[0049] wherein A is at least one element selected from the groupconsisting of Mo and W, D is at least one element selected from thegroup consisting of V and Ce, E is at least one element selected fromthe group consisting of Te, Sb and Se, and X is at least one elementselected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe,Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr,Be, Mg, Ca, Sr, Ba, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu;and a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0, and e is dependenton the oxidation state of the other elements. The catalyst composition,while in its precursor state, is treated with an NO_(x) source, such asnitric acid, and calcined.

[0050] In a fourth aspect of the present invention, a catalystcomposition is provided containing a mixed metal oxide having theempirical formula A_(a)D_(b)E_(c)X_(d)O_(e)

[0051] wherein A is at least one element selected from the groupconsisting of Mo and W, D is at least one element selected from thegroup consisting of V and Ce, E is at least one element selected fromthe group consisting of Te, Sb and Se, and X is at least one elementselected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe,Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr,Be, Mg, Ca, Sr, Ba, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu;and a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0, and e is dependenton the oxidation state of the other elements. The catalyst compositionis treated to exhibit peaks at X-ray diffraction angles (2θ) of 22.1°,27.1°, 28.2°, 36.2°, 45.2°, and 50.0°, with a relative increase in adiffraction peak at the diffraction angle (2θ) of 27.1 degrees whencompared with an untreated catalyst of like empirical formula.

[0052] In this regard, in addition to the above noted peak at 27.1degrees, the preferred mixed metal oxide exhibits the following fivemain diffraction peaks at specific diffraction angles (2θ) in the X-raydiffraction pattern of the treated mixed metal oxide (as measured usingCu-Kα radiation as the source): X-ray lattice plane Diffraction angle 2θSpacing medium Relative (±0.3°) (Å) intensity 22.1° 4.02 100 28.2° 3.16 20˜150 36.2° 2.48  5˜60 45.2° 2.00  2˜40 50.0° 1.82  2˜40

[0053] The intensity of the X-ray diffraction peaks may vary upon themeasuring of each crystal. However, the intensity, relative to the peakintensity at 22.1° being 100, is usually within the above ranges.Generally, the peak intensities at 2θ=22.1° and 28.2° are distinctlyobserved. However, so long as the above five diffraction peaks areobservable, the basic crystal structure is the same even if other peaksare observed in addition to the five diffraction peaks (e.g. at 27.1degrees), and such a structure is useful for the present invention.

[0054] Turning now in more specific detail to the first aspect of thepresent invention, the mixed metal oxide is prepared by treating acatalyst precursor admixture with a source of NO_(x). It will beappreciated that the reference to NO_(x) herein is intended to covercompounds including nitrogen and oxygen, without limitation as to thespecific stoichiometric amounts. However, in a preferred embodiment, xranges up to 3, and more preferably is an integer selected from 1 or 2.

[0055] In a first step, a catalyst precursor admixture may be formed byadmixing metal compounds, preferably at least one of which containsoxygen, and at least one solvent, in appropriate amounts to form theadmixture, which may be a slurry, solution or combination thereof. Asource of NO_(x) is provided and contacted with at least a portion ofthe precursor admixture. Liquids are then removed, and the precursoradmixture calcined.

[0056] More specifically, as mentioned, though a slurry may be formed,preferably, a precursor solution is instead formed at this stage of thecatalyst preparation. Generally, the metal compounds in the solutionwill contain elements A, D, E, X, Y and O, as previously. defined.

[0057] Suitable solvents for the precursor solution include water;alcohols including, but not limited to, methanol, ethanol, propanol, anddiols, etc.; as well as other polar solvents known in the art.Generally, water is preferred. The water is any water suitable for usein chemical syntheses including, without limitation, distilled water andde-ionized water. The amount of water present is preferably an amountsufficient to keep the elements substantially in solution long enough toavoid or minimize compositional and/or phase segregation during thepreparation steps. Accordingly, the amount of water will vary accordingto the amounts and solubilities of the materials combined. Preferably,though lower concentrations of water are possible for forming a slurry,as stated above, the amount of water is sufficient to ensure an aqueoussolution is formed, at the time of mixing.

[0058] The precursor admixture is treated with a source of NO_(x). In apreferred embodiment, the treatment is performed by further admixing theprecursor admixture with a fluid for introducing NO_(x) to the precursoradmixture and then drying or calcining the resulting admixture.Accordingly, preferably the fluid includes a NO_(x) source such asnitric acid, ammonium nitrate, ammonium nitrite, NO, NO₂ or a mixturethereof. More preferably, the fluid is a liquid, such as an aqueoussolution, including the NO_(x) source dissolved or dispersed therein. Inanother embodiment, it is contemplated that a gas including a source ofNO_(x) is bubbled or otherwise introduced into the precursor admixturefor treating the admixture. In a highly preferred embodiment, theprecursor admixture prior to calcination is prepared by mixing theprecursor admixture and nitric acid solution to form a resultingadmixture having 0.01 to 20 percent by weight of nitric acid, and morepreferably 0.05 to 10 percent by weight of nitric acid. In yet anotherpreferred embodiment, the resulting admixture has 0.1 to 1.5 percent byweight of nitric acid. Alternatively expressed, prior to calcination,preferably the nitric acid is present in an amount of at least 500 ppmof the admixture, more preferably, at least 1500 ppm. An example of apreferred range of concentrations includes 1000 to 15,000 ppm nitricacid.

[0059] In another embodiment, where the source of NO_(x) includes NO₂,the amount of NO₂ ranges from 500 to 12,000 ppm and more preferably 1000to 9000 ppm.

[0060] By way of example, when a mixed metal oxide of the formulaMo_(a)V_(b)Te_(c)Nb_(d)O_(e) (wherein the element A is Mo, the element Dis V, the element E is Te and the element X is Nb) is to be prepared, anaqueous solution of niobium oxalate and a solution of aqueous nitricacid may be added to an aqueous solution or slurry of ammoniumheptamolybdate, ammonium metavanadate and telluric acid, so that theatomic ratio of the respective metal elements would be in the prescribedproportions. In one specific illustration, it is further contemplatedthat a 5% aqueous nitric acid is mixed with niobium oxalate solution ina ratio of 1:10 to 1.25:1 parts by volume acid solution to oxalatesolution, and more preferably 1:5 to 1:1 parts by volume acid solutionto oxalate solution.

[0061] Once the resulting NO_(x) treated admixture is formed, the liquidtherein is removed by any suitable method, known in the art, for forminga catalyst precursor. Such methods include, without limitation, vacuumdrying, freeze drying, spray drying, rotary evaporation and air-drying.Vacuum drying is generally performed at pressures ranging from 10 mm Hgto 500 mm Hg. Freeze drying typically entails freezing the slurry orsolution, using, for instance, liquid nitrogen, and drying the frozenslurry or solution under vacuum. Spray drying is generally performedunder an inert atmosphere such as nitrogen or argon, with an inlettemperature ranging from 125° C. to 200° C. and an outlet temperatureranging from 75° C. to 150° C. Rotary evaporation is generally performedat a bath temperature of from 25° C. to 90° C. and at a pressure of from10 mm Hg to 760 mm Hg, preferably at a bath temperature of from 40° to90° C. and at a pressure of from 10 mm Hg to 350 mm Hg, more preferablyat a bath temperature of from 40° C. to 60° C. and at a pressure of from10 mm Hg to 40 mm Hg. Air drying may be effected at temperatures rangingfrom 25° C. to 90° C. Rotary evaporation or air-drying are generallypreferred.

[0062] Once obtained, the resulting catalyst precursor is calcined. Thecalcination may be conducted in an oxygen-containing atmosphere or inthe substantial absence of oxygen, e.g., in an inert atmosphere or invacuo. The inert atmosphere may be any material which is substantiallyinert, i.e., does not react or interact with, the catalyst precursor.Suitable examples include, without limitation, nitrogen, argon, xenon,helium or mixtures thereof. Preferably, the inert atmosphere is argon ornitrogen. The inert atmosphere may flow over the surface of the catalystprecursor or may not flow thereover (a static environment). When theinert atmosphere does flow over the surface of the catalyst precursor,the flow rate can vary over a wide range, e.g., at a space velocity offrom 1 to 500 hr⁻¹.

[0063] The calcination is usually performed at a temperature of from350° C. to 850° C., preferably from 400° C. to 700° C., more preferablyfrom 500° C. to 640° C. The calcination is performed for an amount oftime suitable to form the aforementioned catalyst. Typically, thecalcination is performed for from 0.5 to 30 hours, preferably from 1 to25 hours, more preferably for from 1 to 15 hours, to obtain the desiredpromoted mixed metal oxide.

[0064] In a preferred mode of operation, the catalyst precursor iscalcined in two stages. In the first stage, the catalyst precursor iscalcined in an oxidizing environment (e.g. air) at a temperature of from275° C. to 400° C., preferably from 275° C. to 325° C. for from 15minute 8 hours, preferably for from 1 to 3 hours. In the second stage,the material from the first stage is calcined in a non-oxidizingenvironment (e.g., an inert atmosphere) at a temperature of from 500° C.to 700° C., preferably for from 550° C. to 650° C., for 15 minutes to 8hours, preferably for from 1 to 3 hours. Optionally, a reducing gas,such as, for example, ammonia or hydrogen, may be added during thesecond stage calcination.

[0065] In a particularly preferred mode of operation, the catalystprecursor in the first stage is placed in the desired oxidizingatmosphere at room temperature and then raised to the first stagecalcination temperature and held there for the desired first stagecalcination time. The atmosphere is then replaced with the desirednon-oxidizing atmosphere for the second stage calcination, thetemperature is raised to the desired second stage calcinationtemperature and held there for the desired second stage calcinationtime.

[0066] Although any type of heating mechanism, e.g., a furnace, may beutilized during the calcination, it is preferred to conduct thecalcination under a flow of the designated gaseous environment.Therefore, it is advantageous to conduct the calcination in a bed withcontinuous flow of the desired gas(es) through the bed of solid catalystprecursor particles.

[0067] With calcination, a catalyst is formed having the formulaA_(a)D_(b)E_(c)X_(d)O_(c), wherein A, D, E, X, O, a, b, c, d and e areas previously defined.

[0068] The starting materials for the above promoted mixed metal oxideare not limited to those described above. A wide range of materialsincluding, for example, oxides, nitrates, halides or oxyhalides,alkoxides, acetylacetonates, and organometallic compounds may be used.For example, ammonium heptamolybdate may be utilized for the source ofmolybdenum in the catalyst. However, compounds such as MoO₃, MoO₂,MoCl₅, MoOCl₄, Mo(OC₂H₅)₅, molybdenum acetylacetonate, phosphomolybdicacid and silicomolybdic acid may also be utilized instead of ammoniumheptamolybdate. Similarly, ammonium metavanadate may be utilized for thesource of vanadium in the catalyst. However, compounds such as V₂O₅,V₂O₃, VOCl₃, VCl₄, VO(OC₂H₅)₃, vanadium acetylacet vanadylacetylacetonate may also be utilized instead of ammonium metavanadate.The tellurium source may include telluric acid, TeCl₄, Te(OC₂H₅)₅,Te(OCH(CH₃)₂)₄ and TeO₂. The niobium source may include ammonium niobiumoxalate, Nb2O5, NbCl₅, niobic acid or Nb(OC₂H₅)₅ as well as the moreconventional niobium oxalate. As discussed, without limitation, examplesof preferred NO_(x) sources include nitric acid, ammonium nitrate,ammonium nitrite, NO, NO₂ or a mixture thereof.

[0069] A mixed metal oxide, thus obtained, exhibits excellent catalyticactivities by itself. However, the mixed metal oxide can be converted toa catalyst having higher activities by grinding. There is no particularrestriction as to the grinding method, and conventional methods may beemployed. As a dry grinding method, a method of using a gas streamgrinder may, for example, be mentioned wherein coarse particles arepermitted to collide with one another in a high speed gas stream forgrinding. The grinding may be conducted not only mechanically but alsoby using a mortar or the like in the case of a small scale operation.

[0070] As a wet grinding method wherein grinding is conducted in a wetstate by adding water or an organic solvent to the above mixed metaloxide, a conventional method of using a rotary cylinder-type medium millor a medium-stirring type mill, may be mentioned. The rotarycylinder-type medium mill is a wet mill of the type wherein a containerfor the object to be ground is rotated, and it includes, for example, aball mill and a rod mill. The medium-stirring type mill is a wet mill ofthe type wherein the object to be ground, contained in a container isstirred by a stirring apparatus, and it includes, for example, a rotaryscrew type mill, and a rotary disc type mill.

[0071] The conditions for grinding may suitably be set to meet thenature of the above-mentioned promoted mixed metal oxide, the viscosity,the concentration, etc. of the solvent used in the case of wet grinding,or the optimum conditions of the grinding apparatus. However, it ispreferred that grinding is conducted until the average particle size ofthe ground catalyst precursor would usually be at most 20 μm, morepreferably at most 5 μm. Improvement in the catalytic performance mayoccur due to such grinding.

[0072] Further, in some cases, it is possible to further improve thecatalytic activities by further adding a solvent to the ground catalystprecursor to form a solution or slurry, followed by drying again. Thereis no particular restriction as to the concentration of the solution orslurry, and it is usual to adjust the solution or slurry so that thetotal amount of the starting material compounds for the ground catalystprecursor is from 10 to 60 wt %. Then, this solution or slurry is driedby a method such as spray drying, freeze drying, evaporation to drynessor vacuum drying, preferably by the spray drying method. Further,similar drying may be conducted also in the case where wet grinding isconducted.

[0073] The oxide obtained by the above-mentioned method may be used as afinal catalyst, but it may further be subjected to heat treatmentusually at a temperature of from 200° to 700° C. for from 0.1 to 10hours.

[0074] The resulting mixed metal oxide may be used by itself as a solidcatalyst. It also may be formed into a catalyst with a suitable carrieraccording to art-disclosed techniques. Further, it may be processed to asuitable shape or particle size using art disclosed techniques,depending upon the scale or system of the reactor.

[0075] Turning now in more specific detail to the second aspect of thepresent invention, the present invention provides a process forproducing an unsaturated carboxylic acid, which comprises subjecting analkane, or a mixture of an alkane and an alkene (“alkane/alkene”), to avapor phase catalytic oxidation reaction in the presence of a catalystcontaining the above promoted mixed metal oxide, to produce anunsaturated carboxylic acid.

[0076] In the production of such an unsaturated carboxylic acid, it ispreferred to employ a starting material gas that contains steam. In sucha case, as a starting material gas to be supplied to the reactionsystem, a gas mixture comprising a steam-containing alkane, or asteam-containing mixture of alkane and alkene, and an oxygen-containinggas, is usually used. However, the steam-containing alkane, or thesteam-containing mixture of alkane and alkene, and the oxygen-containinggas may be alternately supplied to the reaction system. The steam to beemployed may be present in the form of steam gas in the reaction system,and the manner of its introduction is not particularly limited.

[0077] Further, as a diluting gas, an inert gas such as nitrogen, argonor helium may be supplied. The molar ratio (alkane or mixture of alkaneand alkene): (oxygen):(diluting gas):(H₂O) in the starting material gasis preferably (1):(0.1 to 10):(0 to 20):(0.2 to 70), more preferably(1):(1 to 5.0):(0 to 10):(5 to 40).

[0078] When steam is supplied together with the alkane, or the mixtureof alkane and alkene, as starting material gas, the selectivity for anunsaturated carboxylic acid is distinctly improved, and the unsaturatedcarboxylic acid can be obtained from the alkane, or mixture of alkaneand alkene, in good yield simply by contacting in one stage. However,the conventional technique utilizes a diluting gas such as nitrogen,argon or helium for the purpose of diluting the starting material. Assuch a diluting gas, to adjust the space velocity, the oxygen partialpressure and the steam partial pressure, an inert gas such as nitrogen,argon or helium may be used together with the steam.

[0079] As the starting material alkane it is preferred to employ a C₃₋₈alkane, particularly propane, isobutane or n-butane; more preferably,propane or isobutane; most preferably, propane. According to the presentinvention, from such an alkane, an unsaturated carboxylic acid such asan α,β-unsaturated carboxylic acid can be obtained in good yield. Forexample, when propane or isobutane is used as the starting materialalkane, acrylic acid or methacrylic acid will be obtained, respectively,in good yield.

[0080] In the present invention, as the starting material mixture ofalkane and alkene, it is preferred to employ a mixture of C₃₋₈ alkaneand C₃₋₈ alkene, particularly propane and propene, isobutane andisobutene or n-butane and n-butene. As the starting material mixture ofalkane and alkene, propane and propene or isobutane and isobutene aremore preferred. Most preferred is a mixture of propane and propene.According to the present invention, from such a mixture of an alkane andan alkene, an unsaturated carboxylic acid such as an α,β-unsaturatedcarboxylic acid can be obtained in good yield. For example, when propaneand propene or isobutane and isobutene are used as the starting materialmixture of alkane and alkene, acrylic acid or methacrylic acid will beobtained, respectively, in good yield. Preferably, in the mixture ofalkane and alkene, the alkene is present in an amount of at least 0.5%by weight, more preferably at least 1.0% by weight to 95% by weight;most preferably, 3% by weight to 90% by weight.

[0081] As an alternative, an alkanol, such as isobutanol, which willdehydrate under the reaction conditions to form its correspondingalkene, i.e. isobutene, may also be used as a feed to the presentprocess or in conjunction with the previously mentioned feed streams.

[0082] The purity of the starting material alkane is not particularlylimited, and an alkane containing a lower alkane such as methane orethane, air or carbon dioxide, as impurities, may be used without anyparticular problem. Further, the starting material alkane may be amixture of various alkanes. Similarly, the purity of the startingmaterial mixture of alkane and alkene is not particularly limited, and amixture of alkane and alkene containing a lower alkene such as ethene, alower alkane such as methane or ethane, air or carbon dioxide, asimpurities, may be used without any particular problem. Further, thestarting material mixture of alkane and alkene may be a mixture ofvarious alkanes and alkenes.

[0083] There is no limitation on the source of the alkene. It may bepurchased, per se, or in admixture with an alkane and/or otherimpurities. Alternatively, it can be obtained as a by-product of alkaneoxidation. Similarly, there is no limitation on the source of thealkane. It may be purchased, per se, or in admixture with an alkeneand/or other impurities. Moreover, the alkane, regardless of source, andthe alkene, regardless of source, may be blended as desired.

[0084] The detailed mechanism of the oxidation reaction of the presentinvention is not clearly understood, but the oxidation reaction iscarried out by oxygen atoms present in the above mixed metal oxide or bymolecular oxygen present in the feed gas. To incorporate molecularoxygen into the feed gas, such molecular oxygen may be pure oxygen gas.However, it is usually more economical to use an oxygen-containing gassuch as air, since purity is not particularly required.

[0085] It is also possible to use only an alkane, or a mixture of alkaneand alkene, substantially in the absence of molecular oxygen for thevapor phase catalytic reaction. In such a case, it is preferred to adopta method wherein a part of the catalyst is appropriately withdrawn fromthe reaction zone from time to time, then sent to an oxidationregenerator, regenerated and then returned to the reaction zone forreuse. As the regeneration method of the catalyst, a method may, forexample, be mentioned which comprises contacting an oxidative gas suchas oxygen, air or nitrogen monoxide with the catalyst in the regeneratorusually at a temperature of from 300° to 600° C.

[0086] The second aspect of the present invention will be described instill further detail with respect to a case where propane is used as thestarting material alkane and air is used as the oxygen source. Thereaction system may be preferably a fixed bed system. The proportion ofair to be supplied to the reaction system is important for theselectivity for the resulting acrylic acid, and it is usually at most 25moles, preferably from 0.2 to 18 moles per mole of propane, whereby highselectivity for acrylic acid can be obtained. This reaction can beconducted usually under atmospheric pressure, but may be conducted undera slightly elevated pressure or slightly reduced pressure. With respectto other alkanes such as isobutane, or to mixtures of alkanes andalkenes such as propane and propene, the composition of the feed gas maybe selected in accordance with the conditions for propane.

[0087] Typical reaction conditions for the oxidation of propane orisobutane to acrylic acid or methacrylic acid may be utilized in thepractice of the present invention. The process may be practiced in asingle pass mode (only fresh feed is fed to the reactor) or in a recyclemode (at least a portion of the reactor effluent is returned to thereactor). General conditions for the process of the present inventionare as follows: the reaction temperature can vary from 200° C. to 700°C., but is usually in the range of from 200° C. to 550° C., morepreferably 250° C. to 480° C., most preferably 300° C. to 400° C.; thegas space velocity, SV, in the vapor phase reaction is usually within arange of from 100 to 10,000 hr⁻¹, preferably 300 to 6,000 hr⁻¹, morepreferably 300 to 2,000 hr⁻¹; the average contact time with the catalystcan be from 0.01 to 10 seconds or more, but is usually in the range offrom 0.1 to 10 seconds, preferably from 0.2 to 6 seconds; the pressurein the reaction zone usually ranges from 0 to 75 psig, but is preferablyno more than 50 psig. In a single pass mode process, it is preferredthat the oxygen be supplied from an oxygen-containing gas such as air.The single pass mode process may also be practiced with oxygen addition.In the practice of the recycle mode process, oxygen gas by itself is thepreferred source so as to avoid the build up of inert gases in thereaction zone.

[0088] Of course, in the oxidation reaction of the present invention, itis important that the hydrocarbon and oxygen concentrations in the feedgases be maintained at the appropriate levels to minimize or avoidentering a flammable regime within the reaction zone or especially atthe outlet of the reactor zone. Generally, it is preferred that theoutlet oxygen levels be low to both minimize after-burning and,particularly, in the recycle mode of operation, to minimize the amountof oxygen in the recycled gaseous effluent stream. In addition,operation of the reaction at a low temperature (below 450° C.) isextremely attractive because after-burning becomes less of a problemwhich enables the attainment of higher selectivity to the desiredproducts. The catalyst of the present invention operates moreefficiently at the lower temperature range set forth above,significantly reducing the formation of acetic acid and carbon oxides,and increasing selectivity to acrylic acid. As a diluting gas to adjustthe space velocity and the oxygen partial pressure, an inert gas such asnitrogen, argon or helium may be employed.

[0089] When the oxidation reaction of propane, and especially theoxidation reaction of propane and propene, is conducted by the method ofthe present invention, carbon monoxide, carbon dioxide, acetic acid,etc. may be produced as by-products, in addition to acrylic acid.Further, in the method of the present invention, an unsaturated aldehydemay sometimes be formed depending upon the reaction conditions. Forexample, when propane is present in the starting material mixture,acrolein may be formed; and when isobutane is present in the startingmaterial mixture, methacrolein may be formed. In such a case, such anunsaturated aldehyde can be converted to the desired unsaturatedcarboxylic acid by subjecting it again to the vapor phase catalyticoxidation with the promoted mixed metal oxide-containing catalyst of thepresent invention or by subjecting it to a vapor phase catalyticoxidation reaction with a conventional oxidation reaction catalyst foran unsaturated aldehyde.

[0090] Turning now in more specific detail to the third aspect of thepresent invention, the method of the present invention comprisessubjecting an alkane, or a mixture of an alkane and an alkene, to avapor phase catalytic oxidation reaction with ammonia in the presence ofa catalyst containing the above mixed metal oxide, to produce anunsaturated nitrile.

[0091] In the production of such an unsaturated nitrile, as the startingmaterial alkane, it is preferred to employ a C₃₋₈ alkane such aspropane, butane, isobutane, pentane, hexane and heptane. However, inview of the industrial application of nitrites to be produced, it ispreferred to employ a lower alkane having 3 or 4 carbon atoms,particularly propane and isobutane.

[0092] Similarly, as the starting material mixture of alkane and alkene,it is preferred to employ a mixture of C₃₋₈ alkane and C₃₋₈ alkene suchas propane and propene, butane and butene, isobutane and isobutene,pentane and pentene, hexane and hexene, and heptane and heptene.However, in view of the industrial application of nitrites to beproduced, it is more preferred to employ a mixture of a lower alkanehaving 3 or 4 carbon atoms and a lower alkene having 3 or 4 carbonatoms, particularly propane and propene or isobutane and isobutene.Preferably, in the mixture of alkane and alkene, the alkene is presentin an amount of at least 0.5% by weight, more preferably at least 1.0%by weight to 95% by weight, most preferably 3% by weight to 90% byweight.

[0093] The purity of the starting material alkane is not particularlylimited, and an alkane containing a lower alkane such as methane orethane, air or carbon dioxide, as impurities, may be used without anyparticular problem. Further, the starting material alkane may be amixture of various alkanes. Similarly, the purity of the startingmaterial mixture of alkane and alkene is not particularly limited, and amixture of alkane and alkene containing a lower alkene such as ethene, alower alkane such as methane or ethane, air or carbon dioxide, asimpurities, may be used without any particular problem. Further, thestarting material mixture of alkane and alkene may be a mixture ofvarious alkanes and alkenes.

[0094] There is no limitation on the source of the alkene. It may bepurchased, per se, or in admixture with an alkane and/or otherimpurities. Alternatively, it can be obtained as a by-product of alkaneoxidation. Similarly, there is no limitation on the source of thealkane. It may be purchased, per se, or in admixture with an alkeneand/or other impurities. Moreover, the alkane, regardless of source, andthe alkene, regardless of source, may be blended as desired.

[0095] The detailed mechanism of the ammoxidation reaction of thisaspect of the present invention is not clearly understood. However, theoxidation reaction is conducted by the oxygen atoms present in the abovepromoted mixed metal oxide or by the molecular oxygen in the feed gas.When molecular oxygen is incorporated in the feed gas, the oxygen may bepure oxygen gas. However, since high purity is not required, it isusually economical to use an oxygen-containing gas such as air.

[0096] As the feed gas, it is possible to use a gas mixture comprisingan alkane, or a mixture of an alkane and an alkene, ammonia and anoxygen-containing gas, However, a gas mixture comprising an alkane or amixture of an alkane and an alkene and ammonia, and an oxygen-containinggas may be supplied alternately.

[0097] When the gas phase catalytic reaction is conducted using analkane, or a mixture of an alkane and an alkene, and ammoniasubstantially free from molecular oxygen, as the feed gas, it isadvisable to employ a method wherein a part of the catalyst isperiodically withdrawn and sent to an oxidation regenerator forregeneration, and the regenerated catalyst is returned to the reactionzone. As a method for regenerating the catalyst, a method may bementioned wherein an oxidizing gas such as oxygen, air or nitrogenmonoxide is permitted to flow through the catalyst in the regeneratorusually at a temperature of from 300° C. to 600° C.

[0098] The third aspect of the present invention will be described infurther detail with respect to a case where propane is used as thestarting material alkane and air is used as the oxygen source. Theproportion of air to be supplied for the reaction is important withrespect to the selectivity for the resulting acrylonitrile. Namely, highselectivity for acrylonitrile is obtained when air is supplied within arange of at most 25 moles, particularly 1 to 15 moles, per mole of thepropane. The proportion of ammonia to be supplied for the reaction ispreferably within a range of from 0.2 to 5 moles, particularly from 0.5to 3 moles, per mole of propane. This reaction may usually be conductedunder atmospheric pressure, but may be conducted under a slightlyincreased pressure or a slightly reduced pressure. With respect to otheralkanes such as isobutane, or to mixtures of alkanes and alkenes such aspropane and propene, the composition of the feed gas may be selected inaccordance with the conditions for propane.

[0099] The process of the third aspect of the present invention may beconducted at a temperature of, for example, from 250° C. to 480° C. Morepreferably, the temperature is from 300° C. to 400° C. The gas spacevelocity, SV, in the gas phase reaction is usually within the range offrom 100 to 10,000 hr⁻¹, preferably from 300 to 6,000 hr⁻¹, morepreferably from 300 to 2,000 hr⁻¹. As a diluent gas, for adjusting thespace velocity and the oxygen partial pressure, an inert gas such asnitrogen, argon or helium can be employed. When ammoxidation of propaneis conducted by the method of the present invention, in addition toacrylonitrile, carbon monoxide, carbon dioxide, acetonitrile,hydrocyanic acid and acrolein may form as by-products.

[0100] Turning now in more specific detail to the fourth aspect of thepresent invention, the treated catalyst exhibits peaks at diffractionangles (2θ) of 22.1°, 27.1°, 28.2°, 36.2°, 45.2°, and 50.0°. As comparedwith an untreated catalyst composition, the treated catalyst compositionof the present invention exhibits an X-ray diffraction pattern having arelative increase in a diffraction peak at a diffraction angle (2θ) of27.1 degrees when compared with an untreated catalyst, which may exhibitno peak at all at 27.1 degrees.

[0101] The relative difference between peak intensities of treatedversus untreated compositions may be greater than 5%, more preferablygreater than 10%, and still more preferably greater than 20% of theintensity of the untreated catalyst composition at the diffraction angle(2θ) of 27.1 degrees. Without intending to be bound by theory, it isbelieved that at least two phases (A and B) are present in the resultingmixed metal oxide catalyst and the treatment of the catalyst precursorwith a source of NO_(x) results in an increase in phase B relative tophase A in the resulting catalyst. The increase in phase B is believedto contribute to improved performance of the catalyst in terms ofselectivity, reactivity and yield.

EXAMPLES

[0102] Catalyst Preparation

Example 1

[0103] 100 mL of an aqueous solution containing ammonium heptamolybdatetetrahydrate (1.0 M Mo), ammonium metavanadate (0.3 M V) and telluricacid (0.23 M Te), formed by dissolving the corresponding salts in waterat 70° C., was added to a 1000 mL rotavap flask. Then 50 mL of anaqueous solution of niobium oxalate (0.25M Nb) and oxalic acid (0.3 1M)were added thereto. After removing the water via a rotary evaporatorwith a warm water bath at 50° C. and 28 mm/Hg, the solid materials werefurther dried in a vacuum oven at 25° C. overnight and then calcined.Calcination was effected by placing the solid materials in an airatmosphere and then heating them to 275° C. at 10° C./min and holdingthem under the air atmosphere at 275° C. for one hour; the atmospherewas then changed to argon and the material was heated from 275° C. to600° C. at 2° C./min and the material was held under the argonatmosphere at 600° C. for two hours. The final catalyst had a nominalcomposition of Mo₁V_(0.3)Te_(0.23)Nb_(0.125)O_(f). The catalyst, thusobtained, was pressed in a mold and then broken and sieved to 10 -20mesh granules for reactor evaluation.

Example 2

[0104] 100 mL of an aqueous solution containing ammonium heptamolybdatetetrahydrate (1.0 M Mo), ammonium metavanadate (0.3 M V) and telluricacid (0.23 M Te), formed by dissolving the corresponding salts in waterat 70° C., was added to a 1000 mL rotavap flask. Then 10 mL of 5%aqueous HNO₃ and 50 mL of an aqueous solution of niobium oxalate (0.25 MNb) and oxalic acid (0.31 M) were added thereto. After removing thewater via a rotary evaporator with a warm water bath at 50° C. and 28mm/Hg, the solid materials were further dried in a vacuum oven at 25° C.overnight and then calcined. Calcination was effected by placing thesolid materials in an air atmosphere and then heating them to 275° C. at10° C./min and holding them under the air atmosphere at 275° C. for onehour; the atmosphere was then changed to argon and the material washeated from 275° C. to 600° C. at 2° C./min and the material was heldunder the argon atmosphere at 600° C. for two hours. The final catalysthad a nominal composition of Mo₁V_(0.3)Te_(0.23) Nb_(0.125)O_(f). Thecatalyst, thus obtained, was pressed in a mold and then broken andsieved to 10 -20 mesh granules for reactor evaluation.

Example 3

[0105] 100 mL of an aqueous solution containing ammonium heptamolybdatetetrahydrate (1.0 M Mo), ammonium metavanadate (0.3 M V) and telluricacid (0.23 M Te), formed by dissolving the corresponding salts in waterat 70° C., was added to a 1000 mL rotavap flask. Then 20 mL of 5%aqueous HNO₃ and 50 mL of an aqueous solution of niobium oxalate (0.25 MNb) and oxalic acid (0.31 M) were added thereto. After removing thewater via a rotary evaporator with a warm water bath at 50° C. and 28mm/Hg, the solid materials were further dried in a vacuum oven at 25° C.overnight and then calcined. Calcination was effected by placing thesolid materials in an air atmosphere and then heating them to 275° C. at10° C./min and holding them under the air atmosphere at 275° C. for onehour; the atmosphere was then changed to argon and the material washeated from 275° C. to 600° C. at 2° C./min and the material was heldunder the argon atmosphere at 600° C. for two hours. The final catalysthad a nominal composition of Mo₁V_(0.3)Te_(0.23)Nb_(0.125)O_(f). Thecatalyst, thus obtained, was pressed in a mold and then broken andsieved to 10 -20 mesh granules for reactor evaluation.

Example 4

[0106] 100 mL of an aqueous solution containing ammonium heptamolybdatetetrahydrate (1.0 M Mo), ammonium metavanadate (0.3 M V) and telluricacid (0.23 M Te), formed by dissolving the corresponding salts in waterat 70° C., was added to a 1000 mL rotavap flask. Then 30 mL of 5%aqueous HNO₃ and 50 mL of an aqueous solution of niobium oxalate (0.25 MNb) and oxalic acid (0.31 M) were added thereto. After removing thewater via a rotary evaporator with a warm water bath at 50° C. and 28mm/Hg, the solid materials were further dried in a vacuum oven at 25° C.overnight and then calcined. Calcination was effected by placing thesolid materials in an air atmosphere and then heating them to 275° C. at10° C./min and holding them under the air atmosphere at 275° C. for onehour; the atmosphere was then changed to argon and the material washeated from 275° C. to 600° C. at 2° C./min and the material was heldunder the argon atmosphere at 600° C. for two hours. The final catalysthad a nominal composition of Mo₁V_(0.3)Te_(0.23) Nb_(0.125)O_(f). Thecatalyst, thus obtained, was pressed in a mold and then broken andsieved to 10 -20 mesh granules for reactor evaluation.

Example 5

[0107] 100 mL of an aqueous solution containing ammonium heptamolybdatetetrahydrate (1.0 M Mo), ammonium metavanadate (0.3 M V) and telluricacid (0.23 M Te), formed by dissolving the corresponding salts in waterat 70° C., was added to a 1000 mL rotavap flask. Then 40 mL of 5%aqueous HNO₃ and 50 mL of an aqueous solution of niobium oxalate (0.25 MNb) and oxalic acid (0.31 M) were added thereto. After removing thewater via a rotary evaporator with a warm water bath at 50° C. and 28mn/Hg, the solid materials were further dried in a vacuum oven at 25° C.overnight and then calcined. Calcination was effected by placing thesolid materials in an air atmosphere and then heating them to 275° C. at10° C./min and holding them under the air atmosphere at 275° C. for onehour; the atmosphere was then changed to argon and the material washeated from 275° C. to 600° C. at 2° C./min and the material was heldunder the argon atmosphere at 600° C. for two hours. The final catalysthad a nominal compostion of Mo₁V_(0.3)Te_(0.23)Nb_(0.125)O^(f). Thecatalyst, thus obtained, was pressed in a mold and then broken andsieved to 10 -20 mesh granules for reactor evaluation.

Example 6

[0108] 100 mL of an aqueous solution containing ammonium heptamolybdatetetrahydrate (1.0 M Mo), ammonium metavanadate (0.3 M V) and telluricacid (0.23 M Te), formed by dissolving the corresponding salts in waterat 70° C., was added to a 1000 mL rotavap flask. Then 50 mL of 5% HNO₃and 50 mL of an aqueous solution of niobium oxalate (0.25 M Nb) andoxalic acid (0.31 M) were added thereto. After removing the water via arotary evaporator with a warm water bath at 50° C. and 28 mm/Hg, thesolid materials were further dried in a vacuum oven at 25° C. overnightand then calcined. Calcination was effected by placing the solidmaterials in an air atmosphere and then heating them to 275° C. at 10°C./min and holding them under the air atmosphere at 275° C. for onehour; the atmosphere was then changed to argon and the material washeated from 275° C. to 600° C. at 2° C./min and the material was heldunder the argon atmosphere at 600° C. for two hours). The final catalysthad a nominal compostion of Mo₁V_(0.3)Te_(0.23)Nb_(0.125)O^(f). Thecatalyst, thus obtained, was pressed in a mold and then broken andsieved to 10 -20 mesh granules for reactor evaluation.

[0109] Evaluation and Results

[0110] Catalysts were evaluated in a 10 cm long Pyrex tube reactor(internal diameter: 3.9 mm). The catalyst bed (4 cm long) was positionedwith glass wool at approximately mid-length in the reactor and washeated with an electric furnace. Mass flow controllers and metersregulated the gas flow rate. The oxidation was conducted using a feedgas stream of propane, stream and air, with a feed ratio ofpropane:steam:air of 1:3:96. The reactor effluent was analyzed by anFTIR. The results at a 3 second residence time are shown in Table 1.TABLE 1 Example Temp. C. % C3 Conv. % AA yield 1 390 41 17 2 350 63 34 3381 46 34 4 373 55 40 5 389 56 43 6 373 48 37

[0111] As gleaned from the above, the present catalyst compositions,when treated according to the present invention performs better thanuntreated compositions of like empirical formula. The treated catalystcomposition exhibits at least 1.5, and more preferably 2, times theyield of reaction product as compared with an untreated catalystcomposition of like empirical formula.

[0112] A relative improvement in the conversion of the gaseous reactant(e.g., alkane or alkene) of at least 10%, and more preferably at least20% is observed using the treated composition of the present invention,as compared with an untreated composition under like processingconditions.

[0113] All publications discussed in the foregoing are hereby expresslyincorporated by reference herein for all purposes. Catalysts disclosedtherein may also be treated using the techniques of the presentinvention.

What is claimed is:
 1. A process for improving the performancecharacterisitics of a catalyst, comprising the steps of: a) providingprecursors for a mixed metal oxide having the empirical formulaA_(a)D_(b)E_(c)X_(d)O_(e), wherein A is at least one element selectedfrom the group consisting of Mo and W, D is at least one elementselected from the group consisting of V and Ce, E is at least oneelement selected from the group consisting of Te, Sb and Se, and X is atleast one element selected from the group consisting of Nb, Ta, Ti, Al,Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na,K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Hf, Ag, Pb, P, Pm, Eu, Gd, Dy, Ho,Er, Tm, Yb and Lu; and a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0,and e is dependent on the oxidation state of said other elements; b)adding a source of NO_(x) to said precursors to form an admixture; andc) calcining said admixture while said NO_(x) is present in saidadmixture.
 2. The process according to claim 1, wherein said source ofNO_(x) is selected from nitric acid, ammonium nitrate, ammonium nitrite,NO, NO₂ or a mixture thereof.
 3. The process according to claim 1,wherein said source of NO_(x) is nitric acid.
 4. The process accordingto claim 1, wherein said providing step (a) includes forming anadmixture of metal compounds, at least one of which contains oxygen, inat least one solvent.
 5. The process according to claim 1, wherein saidadding step (b) includes adding said source of NO_(x) in a gaseousstate.
 6. A process for producing an unsaturated carboxylic acid, whichcomprises subjecting an alkane or a mixture of an alkane and an alkeneto a vapor phase catalytic oxidation reaction in the presence of acatalyst containing a mixed metal oxide having the empirical formulaA_(a)D_(b)E_(c)X_(d)O_(e) wherein A is at least one element selectedfrom the group consisting of Mo and W, D is at least one elementselected from the group consisting of V and Ce, E is at least oneelement selected from the group consisting of Te, Sb and Se, and X is atleast one element selected from the group consisting of Nb, Ta, Ti, Al,Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na,K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Hf, Ag, Pb, P, Pm, Eu, Gd, Dy, Ho,Er, Tm, Yb and Lu; and a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0,and e is dependent on the oxidation state of said other elements, saidcatalyst composition having been formed from calcining an admixtureincluding catalyst precursors and a source of NO_(x) for improvingcatalytic performance.
 7. The process according to claim 6, wherein saidsource of NO_(x) is selected from nitric acid, ammonium nitrate,ammonium nitrite, NO, NO₂ or a mixture thereof.
 8. The process accordingto claim 6, wherein said source of NO_(x) is nitric acid.
 9. A processfor producing an unsaturated nitrite, which comprises subjecting analkane, or a mixture of an alkane and an alkene, and ammonia to a vaporphase catalytic oxidation reaction in the presence of a catalystcontaining a mixed metal oxide having the empirical formulaA_(a)D_(b)E_(c)X_(d)O_(e) wherein A is at least one element selectedfrom the group consisting of Mo and W, D is at least one elementselected from the group consisting of V and Ce, E is at least oneelement selected from the group consisting of Te, Sb and Se, and X is atleast one element selected from the group consisting of Nb, Ta, Ti, Al,Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na,K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Hf, Ag, Pb, P, Pm, Eu, Gd, Dy, Ho,Er, Tm, Yb and Lu; and a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0,and e is dependent on the oxidation state of said other elements, saidcatalyst composition having been formed from calcining an admixtureincluding catalyst precursors and a source of NO_(x) for improvingcatalytic performance.
 10. The process according to claim 9, whereinsaid source of NO_(x) is selected from nitric acid, ammonium nitrate,ammonium nitrite, NO, NO₂ or a mixture thereof.
 11. The processaccording to claim 9, wherein said source of NO_(x) is nitric acid. 12.An improved catalyst composition, comprising: a mixed metal oxide havingthe empirical formula A_(a)D_(b)E_(c)X_(d)O_(e), wherein A is at leastone element selected from the group consisting of Mo and W, D is atleast one element selected from the group consisting of V and Ce, E isat least one element selected from the group consisting of Te, Sb andSe, and X is at least one element selected from the group consisting ofNb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As,Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Hf, Ag, Pb, P, Pm,Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu; and a=1, b=0.01 to 1.0, c=0.01 to1.0, d=0.01 to 1.0, and e is dependent on the oxidation state of saidother elements; wherein said catalyst composition has been treated toexhibit peaks at X-ray diffraction angles (2θ) of 22.1°, 27.1°, 28.2°,36.2°, 45.2°, and 50.0°, with a relative increase in a diffraction peakat said diffraction angle (2θ) of 27.1 degrees when compared with anuntreated catalyst of like empirical formula.